Transmission system



April 2, 1947.

E. LABIN 2,419,568

TRANSMISSION SYSTEM Filed 4July 16, 1943 3 Sheets-Sheet 1 /16 A/I TRANSMITTER- April 29, E. LABIN TRANSMISSION SYSTEM Filed July 16, 1945 3 Sheets-Sheet 2 MSTER W4I/E 52 10a) /rfr Smm /Mvur 2o N 10 y Ewa QZ@ -4 *y TRANSMITTER /Id 75- 22a 76. l /20 P/cn/RE Pawf GEM .Sm/wwwa AMR AMR 8 oir/c5 Manz/Mw@ scn/ww/vs l2 MAsrE/P WAVE M11/E G51/imma? @s1/IRAM@ IN V EN TOR. f/W/l f H5/N AITIRNY 'April 29, 1947.

E. LABIN TRANSMISSION SYSTEM F-led July 16, 1945 3 Sheets-Sheet 3 INVENTOR. f'M/E H5/N Patented Apr. 29, 1947 UNITED STATES 2,419,568 TRANSMISSION SYSTEM Emile Labin, New York, N. Y., assignor to Federal Telephone and Radio Corporation, Newark, N. .1., a corporation of Delaware Application July 16, 1943, Serial No. 494,924

(Cl. Z50-.6)

12 Claims.

This invention relates to secret signalling systems in which speech or other intelligence signals are masked for transmission at the sending station by means of a selected key signal and are unmasked at the receiving station by removal of the masking component of the received signal.

The practice of masking messages by modifying an intelligence signal by a key signal has been employed heretofore for transmission of secret messages. In such systems heretofore proposed, however, some special form of synchronization was necessary in order properly to remove the masking component of the received signal.

It is one of the objects of this invention to provide a method and means for transmitting masked messages between stations by wire or wireless without the requirement of special synchronizing signals for controlling the unmasking of the re ceived signals.

Another object of this invention is to provide a transmitting system and method utilizing a transmitting medium capable of conveying the masked message wherein the transmitting medium has a timing characteristic which is indicative of the timing of the masking signal and which may be used for synchronizing an unmasking receiver.

Still another object of the invention is to provide a receiver and method for receiving masked message from a transmission medium the timing characteristic of which is indicative of the timing of the masking signal component at the transmitting station and wherein the receiver utilizes the timing characteristic of the carrying medium for synchronizing the key signal at the receiving station for removing the masking signal component from the signals received.

According to one feature of the present invention,` a transmission medium, such as a train of pulses, is selected having a timing characteristic which can be used for synchronizing the generation of an unmasking signal at the receiver station with the masking key signal applied at the sending station. The expression transmission medium is here used with reference to the formation of the medium conveying the signals whether or not a radio frequency carrier wave is employed. For example, where wire transmission is used, the R.F. carrier Wave may be dispensed with, if desired, and the train of pulses which are modulated with the signal energy, applied directly to the transmission line. The energy of intelligence signals and of masking signals usedas the modulating energy for the transmissionV medium may be applied to the pulses of the medium by any one ofrseveral different modulating methods. Time modulation of pulses, for example, may take any one of several forms, each such form requiring a train of pulses having a given repetition rate at least as high as and normally higher than the highest frequency component in the signal wave to be conveyed.. In the simplest examples, every pulse has the same amplitude. In single pulse time modulation, the modulation of the pulses is represented by variation of the duration or width of each pulse, such as by varying the occurrence of the leading or trailing edge with respect to the other, linearly in proportion to the instantaneous amplitude ci the signal wave. The single pulse method may be translated into still another different time modulation form if desired. That is to say, each of the varying width pulses may be translated by a differentiating and inverting process into two constant width pulses one corresponding to the leading edge of the signal pulse and the other corresponding to the trailing edge thereof. This provides two series of pulses, the pulses of one series having a xed timing while the pulses of the second series, which are positioned alternately with the pulses of the first-mentioned series, are time displaced with respect to the adjacent pulses of said rst-mentioned series, and, if desired may be eliminated.

Another method of time modulation is double pulse modulation wherein the modulation is performed by pairs of pulses. The pulses in this example are of constant duration and small compared with the time interval between adjacent pulses. The train of double modulated pulses may be made up of pulses having a given repetition rate wherein the time interval between the pulses of each pair for such rate represent the one limit of signal amplitude swing in onev direction and the pulse displacement from such repetition rate represents the degree of signal amplitude variation from said one limit. Thusthe displacement of the pulses towardl or away from each other with respect to their normal repetition positioning is proportional to the instantaneous amplitude of the signal wave. This example, of course, is but one of several variations of double pulse modulation.

Besides the abode modulation methods, amplitude modulation may also be applied to pulses of a given repetition rate for use in the secret signalling principles of this invention. In fact, different channels of intelligence may be provided by combining forms of time modulation with amplitude modulation.

According to the principles of the present invention, a great latitude of selection of the masking signal is possible. The speech or other intelligence signal to be transmitted is modied by adding thereto the energy of the masking signal thereby producing a complex wave so masking the intelligence signal with any attempt to detect the complex wave by ordinary receiver means results in a distorted reception. A simple masking signal for this purpose may be produced by scanning a given key picture by use of a simple form of television camera synchronized by a master wave with the timing characteristic of the transmission medium. The signal energy generated by the scanning device is added algebraically to the intelligence signal to produce the complex wave. At the receiving station an exact copy of the given key picture is scanned by a scanning device synchronized to the characteristic timing of the transmission medium. This secondscanning device generates an unmasking signalwhich is opposite in polarity to the masking signal and when the unmasking signal is applied to the received wave the intelligence signalcomponentthereof is unmasked.

In order for the masking signal to provide an audible tone capable of adequately distorting a speech signal, the number of scanning lines and frames scanned must be so chosen as to generate a signal including mainly the frequencies of the audio spectrum. 600 lines and 60 frames, for example, when scanned per second provide a lowerI masking signal frequency of 60 cycles per second and a higher frequency at approximately 600 per second. Still higher frequency compo nents may be generated depending upon the characteristic of the picture used as a key. At thisV speed of scanning, a usual cathode ray scanning tube may be used as the scanning device and if a lm is used as a key picture, the number Aof possible combinations can be varied widely conforming, of course, to a predetermined cycle.

For a further understanding of the invention, reference may be had to the following detailed description to-be read in connection with the accompanying drawings, in which:

Fig. 1 is a block diagram of a transmitter station according to the principles of this invention;

Fig. 2 is a schematic wiring diagram of the pulse generator and modulator device of Fig. l;

Fig, 3 is a schematic wiring diagram of a modied arrangement for mixing a key signal with anintelligence signal prior to application to the pulse generator andmodulator device of Fig. 2;

Fig. 4 is a block diagram of a modiiied form of transmitter;

Fig. 5 is a block diagram of a receiving station according to the invention;

Fig. 6` is a block diagram of a modified form of. receiver;

Fig. 7Y is a schematic wiring diagram of the modulating wave generator and demodulator units of Fig. 6; and

Fig. 8Y is a graphical illustration of the demodulation` feature of the receiver of Figs. 6 and '7.

OneY form of secret message transmitter utilizing; pulses as a mediumV of transmission is shown in Fig. 1 wherein the pulse repetition rate iscontrolled by a master wave generator I0; The master wave is also used to control generator I2 for generation of scanning voltages for a scanning device M. The scanning device may be a simpleform of televisionscanning camera adaptedto be focused upon a key picture Iaand by means of acathode ray produce. voltage andcurrent variations according to the black and lwhite and/or color characteristics of the picture for each line I5 scanned by the ray. The scanning voltages generated will comprise a line scanning voltage and a frame scanning voltage the two being correlated in the proportion of say 600 lines and frames per second. Other scanning frequencies, of course, may be used, it being understood that these figures are given only by way of illustration.

The output produced by the scanning device is amplied at i8 and then applied as a modulating source to the pulse generator and modulator 2l). Electrical current variations produced by speech cr other intelligence at microphone 22 are an pliiied at 23 and applied to the pulse generator and modulator device 2i). The device 2f! serves, as will be explained in more detail hereinafter, to generate a train of pulses the repetition rate of which is controlled by the master wave from generator ie and also to modulate the pulses by the joint application of the masking signal from the scanning device M and the intelligence signal from microphone 22; The pulses may or may not need a shaping operation depending on the character of the device 29. A pulse shaper 25, however, is included since it may be desirable to further shape the pulse output of the device 20 before transmission. The signal modulated pulses from the shaper 25 are used to modulate in the usual manner in a modulator 28 the R.-F. energy from source 25. The resulting pulse modulated carrier may then be radiated by the antenna system 3i).

t will be observed that the transmitting system of Fig. 1 may in general be used with any one of the aforementioned forms of time modulation, amplitude modulation and combinations o time and amplitude modulation.

The pulse generator and modulator 20 may comprise separate units for the pulse generating and modulating functions, but, as illustrated in Fig. 2, a single unit may be provided to perform these two functions. The form of pulse generator and modulator of Fig. 2 is substantially the same as one of the forms disclosed in the copending application of E. Labin and D. D. Grieg, Serial No. L155,897, filed August 24, 1942, except where it is shown with two signal input sources.

The circuit of the device 20 includes an input transformer 3l which serves to not only apply 'ie master wave from generator IG for translation by the vacuum tubes 32 and 33, but also to apply thereto the modulating energy of the masking signal and the intelligence signal. For these purposes the transformer has split primary wind.- ing 34 adapted to be connected to the generator I and two auxiliary split windings 36, 31 and 38, 39. The auxiliary windings 36 and 31 receive the masking signal current from the scanning device I4 and the windings 38, 39 receive the intelligence signal current. The primary windings are symmetrically coupled to split secondary windings 4|, i2 so as to deliver in push-pull fashion to the triode vacuum tubes 32 and 33 energy of the master wave together with the modulating energy of the masking and intelligence signals. It will be understood that these vacuum tubes do not actually act as true rectiners, but rather act as translation amplifiers to produce in the output a curve having cusps the spacings of which are substantially proportional to the input over a given range'of input voltages. It will be understood, of course, that while triode vacuum tubes have been selected for this purpose,

'diode rectiers and other forms and arrangement of rectiiiers may be used Aas shown in the aforesaid copending application Serial No. 455,897.

. 'I'he grid circuits of the two vacuum tubes 32 and 33 are biased by batteries 44 and 45, respectively, the biasing value of the tube 33 being about as far below cut-01T as the biasing value for l tube 32 isabove its cut-oil' point. The dilerence Under such conditions, the cusps of the output.

voltage across load resistor 46 will be equally spaced. 'When the modulating input has a maximum polarityin the opposite direction, the difference in potential between the grids of the tubes 3,2 and`33 is accentuated and the cusps of the output voltage Wave across resistor 46 will be displaced by pairs, the cusps of each pair being displaced in opposite directions from the normal unmodulated positions thereof.

' `In operation, the pulse generator and modulator device of Fig. 2 is supplied with a sinusoidal wave Illa from the master generator ||J (Fig. l) in push-pull fashion to the tubes32 and 33. The tube 32 responds to those wave portions extending beyond the threshold level of the tube in` one polarity direction while tube 33 responds to those wave portions extending beyond the threshold thereof in the opposite polarity direction so` that the output now produced by the combined wave portions comprises a voltage curve lllb across the resistor 46 having sharp cuspswhich when no modulating signal is present are substantially equally spaced. Thus, when the signal input varies from one limit toward the opposite limit of variation, the signal input operates to control the translation between balanced to unbalanced operation as indicated by biasing variation between the two levels 5| and 52. This produces a `time displacement of the cusps 5|] as illustrated by the two wave portions 53 and 54 of the curve in Fig. 2.

It will be understood that with the cusps `50 time modulated according to the input signals, a clipping operation at a level such as 50a produces pulses of corresponding time modulation. Such pulses may be further shaped if desired by the pulse shaper in the manner disclosed in the aforesaid copending application Serial No. 455,897.

Assuming that a portion of the masking signal is as represented by curve |5a (Fig. l) and that the corresponding intelligence signal portion is as represented by curve 22a, the combined signal input of these two portions is representable by the curve 20a. By translation in the devices 2U and 25, this complex signal is translated into time modulated pulses, similarly as illustrated at 39a. These narrow pulses may be applied directly to a line or may be used for modulating an R.F. wave as hereinbefore explained.

If desired, the inputV energy of the masking signal and the intelligence signal Vmaybe preliminarily mixed before application to the input transformer 3| of Fig. 2. This preliminary mix- Ving is shown in Fig. 3. Two vacuum tubes A55 and 55 are used in parallel arrangement with the input signals applied to the grids thereof, the masking signal being applied to the grid of tube 55` `and the intelligence signal being applied to thetube 55,. The anodes of the two tubes are connected together and the combined current thereof `passed through windings 51, y58 applied 6 in balanced arrangement upon the input trans former 3|. The anode circuit including the windings 51 and 58 is supplied with a positive potential from a battery terminal 59. Y

In Fig. 4 a further form of transmitter incorporating the principles of this invention is shown. In this form the output of the picture scanning device |4a after being amplified at l5 is applied to the circuit of the microphone 22a. The combined energy of the masking signal from the scanning device and the intelligence signal from the microphone is further'amplied at 16 and applied to the pulse generator and modulator 26a. The picture scanning device is synchronized with the pulse generator and modulator device 2|] by a master wave generator I0 and a scanning wave generator I2 similarly as de scribed in connection with Fig. 1. It will be recognized that in this form of transmitter, the masking signal and intelligence signal are preliminarily mixed before application to the input transformer of device 20, (Fig. 2).

In Fig. 5 a form of receiver is shown by which a. masked signal transmitted by the transmitter of Fig. l whether time or amplitude modulated may be received and the masking signal component removed whereby the intelligence signal may be detected. The receiver includes the usual R.F. detector 60 having an antenna system 6| by which the` carrier wave conveying the secret message may be detected. Since the secret message is conveyed by a transmission medium made up of pulses having a given cadence timing, the energy of the received pulses `are used to generate a synchronizing wave similar to ythe master Wave used in the transmitter. This is accomplished by using a synchronizing circuit 62 which is controlled by the double modulated pulses 30a to produce a synchronizing wave which may be used in turn to control a scanning wave generator B4. A specic form of synchronizer circuit suitable for this purpose is hereinlater described in connection with Fig. 7.

The scanning wave generator 64 is similar to the generator l2 in the transmitter and operates to provide a scanning device 65 with deiiecting voltages substantially identical to those produced for the scanned device I4 in the transmitter of Fig. 1. The key picture 6l' is identical in likeness to the key picture I5. The scanning device 55, however, is arranged to provide an output current according to the scanning operation thereof which is of a polarity opposite that of the current output of the scanning device I4. This unmasking signal component produced 4by the scanning device 65 is amplified at 68 and ap; plied to a mixer device lil. The modulated pulses 30h (which may be assumed here to be time modulated) received by the detector 60 are applied to a demodulator 'Il which for the purposes of illustration may be of any known type, for example, in case the received pulses are time modulated it may be of the character disclosed in the copending application of D. D. Grieg, Serial No. 459,959, filed, September 28, 1942.

In operation, the received pulses 36h are demodulated by demodulator 'll thereby producing an output envelope 20h of the form corresponding to the transmitted complex signal 20a of Fig. l. This envelope energy is applied to the mixer device 1|] which may `comprise a vacuum tube having two input grids one for the inputY of the complex signal energy 20d and the other for the input of the unmasking or negative key .signal |5b. Since the unmasking key signal is applied to the complex output signal 23D with a polarity opposite to that of the original masking signal la, the masking signal component is removed thereby producing at the output of the mixer` 'lo a substantially true reproduction-Zlib of the intelligence signal transmitted. By making the transmitting and receiver systems substantially linear in operation, very little distortion of the intelligence signal will occur on account of the masking` and unmasking operations.

Thereproduced intelligence signal 22h may be detected in the usual manner by application to angaudio amplifier l2 and phones '13.

In case of plural channels where combinations of time and amplitude modulation is used, additional demodulator, mixer and other channel reception stages will be necessary as will be clear to those skilled in the art.

In Fig. 6, a modied form of receiver is shown wherein the unmasking signal is applied to a demodulating wave generator 8l! used in connection with the demodulator stage B2. The transmitted signals are detected by the R.-F. detector 60a and a synchronizing wave is produced from the pulse energy received by generator 62a to control the scanning wave generator 64a which supplies the proper deflection voltages for the picture scanning device 65a. The demodulating Wave generator Bil includes a circuit similar to the synchronizing Wave generator 62a. The generator circuit 80 together with the circuit of the demodulator stage 82 is shown in Fig. '7. The generator` circuit 8i) is provided with a vacuum tube 84 having two grids 85 and 86. yThe input connection 8l for grid 85 is supplied with the time modulated pulses detected at Sila. The connection 88 to grid 86 is supplied with the unmasking signal energy from the scanning device 65a. In the anode circuit 89 is an LC circuit provided with a source of positive voltage at terminal 90.

`Disregardingy for the moment, the input to grid B6, the rest of the circuit of generator 80 comprises the synchronizing wave generator circuit such as may be used at (52l Fig. 5, and 52a, Fig. 6, for response to the received pulses for the production of a sinusoidal wave corresponding to the master wave at the transmitter of Fig. l. The operation of this part of the circuit involves the tuning of the LC circuit preferably to an even harmonic of the fundamental frequency of the received pulses. The received pulses trigger the tube 84 so as to shock excite the LC circuit into oscillations and since the circuit is tunable to the proper even harmonic, a substantially identical wave form to that of the master wave of the transmitter may be producedy in response tothe input pulses.

This synchronizing wave, however, mayV be modified in amplitude by applying the unmasking signal4 produced by the generator 65a to the second grid 86. This unmasking signal voltage `controls the gain of the tube Sil so as tovarythe amplitude of the pulse output of the tube and thereby modify the shocking intensity applied to the LC circuit. Thus, the oscillations of the LC circuit will vary in amplitude according te the variav Thisy is shown also applied from the detector 63a to the control grid 92 of the tube 90 vthrough grid connection 9i.` Since thedemodulating wave 8l has a. timing characteristic synchronized to the period. of the T. M. pulses, the pulse energy is superimposed upon the wave energy and the modulation of the pulses is translated into amplitude modulated pulseienergy minus the masking signal component, thelatter being. introduced by way of the amplitude variations of the demodulating wave. Referring to Fig. 8, assume, for example, that the corresponding portion of the received pulses 30h are time modulated by a given displacement t1.-

These pulses'will then bev superimposed upon the wave potential substantially as indicated. It Will be seen that the resulting envelope 83 is thatofthe complex signal with themasking component removed or, in other words, a reproduction of theV intelligence signal. By suitably biasing the tube 9i]Y to, provide a threshold clipping level above the maximum amplitude of the modulating Wave 8 I, a pulse formed envelope according to the intelligence signal will be produced, which, of course, may be applied to the audio amplifier 95 and phones Elli for normal reception.

.While the demodulating wave generator is shown to include a synchronizing circuit, it will be recognized that the synchronizing feature of this circuit may be used by applying synchronizing wave energy therefrom through a coupling and/or limiting ampliiier stage to the scanning wave generator 64a.. The two synchronizing fea. tures of the invention, however, have been shown separated in order to provide a simple. illustration of the principles of the invention.

While several forms and variations of the numerous features of the invention have been shown and described herein, it will be recognized by those skilled inthe art that manyV additional variations and embodiments may be made without departingV from the invention. It is to be understood, therefore,V that the forms of the invention herein shown and described are to be regarded as illustrativeV of the invention only and not as limiting the objects ofthe invention and the appended claims.

I claim:

l. `A system for secret signalling comprising, at, the sending station, meansto produce a transmission` medium having a given timing characteristicmieans to generate a masking signal from a given key, means utilizing said given time characteristic of said medium to control the generation` of said masking signal, and means to modu-l late said medium according to the instantaneous value of said masking signal andan intelligence signal to be'transmitted, whereby said masking signal operates to mask saidintelligence, signal; and at the receiving station, of means to generate an unmasking signal fromY a key corresponding in likenessto said given key, means responsive to the timing ycharacteristic of said transmission medium to controlthe generation of said unmasking signal, andA means to apply said unmasking signal tdsaid transmission medium to remove therefrom the masking signal component thereof..

, 2. The system defined in. claim l wherein the transmission medium produced includes a train of pulses and the `means for modulating the transmission medium includes means for time displacing the pulses accordingV to theinstantaneous value of the modulating signal energy.`

3. Thelsystem dened in claim 1 wherein the transmissionmedium includes. a train of pulsesl and the meansformodulating the Ytransmission medium includes means for time displacing the pulses by pairs, the pulses of each pair being displaced with respect to each other according to the instantaneous value of the modulating energy.

4. The system defined in claim 1 wherein said given key and said key of corresponding likeness are identical pictures, and the masking and unmasking signal generating means includes means for scanning the key pictures.

5. The system deiined in claim 1 wherein the means for applying said unmasking signal to said transmission medium includes means for demodulating the received pulses, a mixer device, means to apply the output energy of said demodulating means to said mixer device, and means for applying the unmasking signal of polarity opposite that of the masking signal to said mixer device whereby the masking signal component is removed thereby reproducing the intelligence signal.

6. A system for secret signalling comprising, at the sending station, means to produce as a transmission medium a train of pulses having a given repetition rate and which when time modulated by signals retains an average timing cadence substantially equal to said given repetition rate, means to generate a masking signal from a given key, means for synchronizing the generation of said masking signal with the given repetition rate of said pulses, means for mixing energy of said masking signal with energy of an intelligence signal to produce an unintelligible signal for transmission, and means for time modulating said train of pulses according to said `unintelligible signal energy; and at the receiving station, of means to generate an unmasking signal from a key corresponding in likeness to said given key, means responsive to the average cadence timing of the received pulses to synchronize the generation of said unmasking signal, and means utilizing said unmasking signal to remove the masking signal component from the received pulse energy.

7. In a secret signalling system, a transmitter comprising means for producing as a transmission medium a train of pulses having a given repetition rate and which when time modulated by signals retain an average timing cadence substantially equal to said repetition rate so that the average cadence timing can be used for synchronizing an unmasking signal at a receiving station with a given masking signal at the sending station, means to generate a masking signal from a given key, means for synchronizing the generation of the masking signal with the repetition rate of said pulses, and means to time modulate said train of pulses according to the instantaneous values of said masking signal and an intelligence signal to be transmitted.

8. In a system for secret signalling, a transmitter comprising means for producing as a transmission medium a train of pulses, a transmitter comprising means for producing a stable sinusoidal wave, means for translating said sinusoidal wave to produce cusps, means responsive to said sinusoidal wave to control the generation of a masking signal from a given key, means for varying the translation operation between a balanced and a given unbalanced operation according to the energy of an intelligence signal and the energy of said masking signal thereby time modulating the cusps according to the combined energy of the two signals, means for clipping and shaping the cusps to produce time modulated pulses for transmission, the average timing cadence of which can be used at the receiving station for synchronizing the generation of an unmasking signal from a key corresponding in likeness to said given key.

9. The transmitter dened in claim 8 in combination with means for mixing energy of the masking signal with energy of the intelligence signal prior to the modulation thereby of said translation operation.

10. In a secret signalling system, a receiver for receiving an intelligence signal masked by a given key and conveyed thereto by a train of pulses, and wherein the masking component is synchronized to the average cadence timing of the pulses, said receiver comprising means to generate an unmasking signal from a key corresponding in likeness to said given key, means responsive to the average cadence timing of the received pulses to control the operation of the generation of said unmasking signal, and means utilizing the unmasking signal to remove from the energy of the received pulses the masking signal component thereby reproducing said intelligence signal.

11. The receiver dened in claim 10 wherein the means for removing the masking signal component includes a mixer device, means for demodulating the received signals and for applying the demodulated signal wave to said mixer device, and means for applying to said mixer device said unmasking signal of polarity opposite that of the masking signal component whereby the output of said mixer device provides a reproduction of the intelligence signal.

l2. The receiver defined in claim 10 wherein the means for removing the masking signal component includes a demodulator stage, means to generate a demodulating wave in synchronism with the average cadence timing of the received pulses, means utilizing the unmasking signal to amplitude modulate said demodulating wave, and means to apply the received pulses and the amplitude varied demodulating wave to said demodulator stage, whereby demodulation of the'received pulses is obtained with the masking signal component cancelled out by the energy of said unmasking signal.

EMILE LABIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 

